Recently, a design rule of a memory cell has become finer and finer with increasing capacity of a NAND type flash memory. In the generation from a next 2X-nm generation (for example, 24-nm generation), there is a possibility of degrading a characteristic of the memory cell in association with the finer design rule. In reading data, a cell current passed through the memory cell is decreased while a leak current is increased. Therefore, a ratio (hereinafter referred to as an on/off ratio) of a cell current in the case where a read target memory cell (hereinafter referred to as a selected memory cell) is an on-cell and a cell current in the case where the selected memory cell is an off-cell is decreased. The decrease of the on/off ratio reduces a sense margin to increase a possibility of generating a read error. As used herein, the on-cell means a memory cell in a state in which electrons are not accumulated in a floating gate, and the off-cell means a memory cell in a state in which the electrons are accumulated in the floating gate. The on-cell is also referred to as a “1” cell, and the off-cell is also referred to as a “0” cell.
Additionally, with the finer design rule of the memory cell, a power-supply voltage of a sense amplifier tends to be decreased for the purpose of lower power consumption, which also contributes to the reduction of the sense margin.
Conventionally, for example, Japanese Patent Application Laid-Open No. 2001-325796 discloses a semiconductor storage device that includes a capacitor connected to a sense node as a measure against the decrease of the power-supply voltage of the sense amplifier. In the semiconductor storage device disclosed in Japanese Patent Application Laid-Open No. 2001-325796, one end of a capacitor 48c is connected to a sense node N4, and a potential at the other end (BOOST2) of the capacitor 48c is stepped up and stepped down, whereby a potential at the sense node N4 is stepped up and stepped down by capacitance coupling (see paragraphs [0175]-[0192]). However, the potential at the sense node fluctuates not only when the selected memory cell is the on-cell but also when the selected memory cell is the off-cell. Therefore, for the small on/off ratio, the off-cell lacks the sense margin, which possibly results in a problem in that the off-cell is mistakenly read as the on-cell.